Network Working Group A. Karan
Internet-Draft C. Filsfils
Intended status: Informational D. Farinacci
Expires: September 3, 2009 Cisco Systems, Inc.
March 2, 2009
Multicast only Fast Re-Route
draft-karan-mofrr-00
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Abstract
As IPTV deployments grow in number and size, service providers are
looking for solutions that minimize the service disruption due to
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faults in the IP network carrying the packets for these services.
This draft describes a mechanism for minimizing packet loss in a
network when node or link failures occur. Multicast only Fast Re-
Route (MoFRR) works by making simple enhancements to multicast
routing protocols such as PIM.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Conventions used in this document . . . . . . . . . . . . 3
1.2. Terminology . . . . . . . . . . . . . . . . . . . . . . . 3
2. Basic Overview . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Topologies for MoFRR . . . . . . . . . . . . . . . . . . . . . 4
3.1. MoFRR ECMP Topology . . . . . . . . . . . . . . . . . . . 4
4. Detecting Failures . . . . . . . . . . . . . . . . . . . . . . 8
5. ECMP-mode MoFRR . . . . . . . . . . . . . . . . . . . . . . . 8
6. Non-ECMP-mode MoFRR . . . . . . . . . . . . . . . . . . . . . 9
6.1. Variation . . . . . . . . . . . . . . . . . . . . . . . . 10
7. Ring Topologies . . . . . . . . . . . . . . . . . . . . . . . 11
8. Keep It Simple Principle . . . . . . . . . . . . . . . . . . . 11
9. Other Applications . . . . . . . . . . . . . . . . . . . . . . 11
10. Security Considerations . . . . . . . . . . . . . . . . . . . 12
11. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 12
12. References . . . . . . . . . . . . . . . . . . . . . . . . . . 12
12.1. Normative References . . . . . . . . . . . . . . . . . . . 12
12.2. Informative References . . . . . . . . . . . . . . . . . . 12
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 12
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1. Introduction
Multiple techniques have been developed and deployed to improve
service guarantees, both for multicast video traffic and Video on
Demand traffic. Most existing solutions are geared towards finding
an alternate path around one or more failed network elements (link,
node, path failures).
This draft describes a mechanism for minimizing packet loss in a
network when node or link failures occur. Multicast only Fast Re-
Route (MoFRR) works by making simple changes to the way selected
routers use multicast protocols such as PIM. No changes to the
protocols themselves are required. With MoFRR, multicast routing
protocols don't necessarily have to depend on or have to wait on
unicast routing protocols to detect network failures.
MoFRR involves transmitting a multicast join message from a receiver
towards a source on a primary path and transmitting a secondary
multicast join message from the receiver towards the source on a
backup path. Data packets are received from the primary and
secondary paths. The redundant packets are discarded at topology
merge points using RPF checks. When a failure is detected on the
primary path, the repair occurs by changing the interface on which
packets are accepted to the secondary interface. Since the repair is
local, it is fast - greatly improving convergence times in the event
of node or link failures on the primary path.
1.1. Conventions used in this document
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119].
1.2. Terminology
MoFRR : Multicast only Fast Re-Route.
ECMP : Equal Cost Multi-Path.
Primary Join : Multicast join message sent from receiver towards the
source on the primary path.
Secondary Join : Multicast join message sent from receiver towards
the source on the secondary path.
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2. Basic Overview
MoFRR uses standard PIM JOIN/PRUNE messages to set up a primary and a
secondary multicast forwarding path by establishing a primary and a
secondary RPF interface on each router that receives a PIM join. The
outgoing interface list remains the same.
Data packets are received from the primary and backup paths.
Redundant packets received on the secondary RPF interface are
discarded because of an RPF failure. When the router detects a
forwarding failure in the primary path, it changes RPF to the
secondary path and immediately has packets available to forward out
each outgoing interface.
The primary and secondary MoFRR forwarding paths should not use the
same nodes or links. This may be configured or determined by
computations described in this document.
Note, the impact of additional amount of data on the network is
mitigated when group membership is densely populated. When a part of
the network has redundant data flowing, join latency for new joining
members is reduced because joins don't have to propagate far to get
to on-tree routers.
3. Topologies for MoFRR
MoFRR works best in topologies illustrated in the figure below.
MoFRR may be enabled on any router in the network. In the figures
below, MoFRR is shown enabled on the Provider Edge (PE) routers to
illustrate one way in which the technology may be deployed.
3.1. MoFRR ECMP Topology
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S
PJ / \ PJ
/ \
^ G1 R1 ^
PJ / \ PJ
/ \
G2----------R2 ^
| \ | \ PJ
^ | \ | \
PJ | G3----------R3
| | | |
| | | | ^
G4---|------R4 | PJ
^ \ | \ |
PJ \ | \ |
G5----------R5
^ | | ^
PJ | | PJ
| |
Gi Ri
\ \__ ^ /|
\ \ SJ1/ | ^
^ \ ^\ / |PJ2
PJ1 \SJ2\_/__ |
\ / \|
PE1 PE2
PJ = Primary Join
SJ = Secondary Join
FIG1. Two-Plane Network Design
The topology has two planes, a primary plane and a secondary plane
that are fully disjoint from each other all the way into the POPs.
This two plane design is common in service provider networks as it
eliminates single point of failures in their core network. The links
marked PJ indicate the normal path of how the PIM joins flow from the
POPs towards the source of the network. Multicast streams,
especially for the densely watched channels, typically flow along
both the planes in the network anyways.
The only change MoFRR adds to this is on the links marked SJ where
the PE routers send a secondary PIM joins to their ECMP neighbor
towards the source. As a result of this, each PE router receives two
copies of the same stream, one from the primary plane and the other
from the secondary plane. As a result of normal multicast RPF checks
the multicast stream received over the primary path is accepted and
forwarded to the downstream links. The copy of the stream received
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on the secondary path is discarded.
When a router detects a routing failure on its primary RPF interface,
it will switch to the secondary RPF interface and accept packets on
that stream. If the failure is repaired the router may switch back.
The primary and secondary path have only local context and not end-
to-end context.
As one can see, MoFRR achieves the faster convergence by pre-building
the secondary multicast tree and receiving the traffic on that
secondary path. The example discussed above is a simple case where
there are two ECMP paths from each PE device towards the source, one
along the primary plane and one along the secondary. In cases where
the topology is asymmetric or is a ring, this ECMP nature does not
hold, and additional rules have to be taken into account to choose
when and where to send the secondary PIM joins.
MoFRR is appealing in such topologies for the following reasons:
1. Ease of deployment and simplicity: the functionality is only
required on the PE devices although it may be configured on all
routers in the topology. Furthermore, each PE device can be
enabled separately. PEs not enabled for MoFRR do not see any
change or degradation. Inter-operability testing is not required
as there is no PIM protocol change.
2. End-to-end failure detection and recovery: any failure along the
path from the source to the PE can be detected and repaired with
the secondary disjoint stream.
3. Capacity Efficiency: as illustrated in the previous example, the
PIM trees corresponding to IPTV channels cover the backbone and
distribution topology in a very dense manner. As a consequence,
the secondary joins graft into the normal PIM trees (ie. trees
signaled by PIM without MoFRR extension) at the aggregation level
and hence do not demand any extra capacity either on the
distribution links or in the backbone. They simply use the
capacity that is normally used, without any duplication. This is
different from conventional FRR mechanisms which often duplicate
the capacity requirements (the backup path crosses links/nodes
which already carry the primary/normal tree and hence twice as
much capacity is required).
These properties are highly dependent on the topology: the existence
of at least two disjoint paths from the source to the PE implementing
the MoFRR behavior.
We consider hereafter three types of PE connectivity:
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1. The PE has two ECMP disjoint paths to the source. This is a
common case when the PE and the source are dual-homed to two
aggregation routers, each ones belonging to two different planes
of a two-plane backbone design. One could picture this case as
the triangle. ECMP-mode MoFRR enables a PE to receive the same
PIM tree from both interfaces.
Source
/ \
Plane1 Plane2
| |
A1 A2
\ /
PE
FIG2. ECMP-mode MoFRR
2. The PE has one single path to the source. A neighbor has a
disjoint path to that source. This is common when the source is
dual-homed to the two planes while the PE is only connected to
one plane directly, and to the other plane via another PE. One
could picture this case as the square. Non-ECMP-mode MoFRR
enables a PE to receive the same PIM tree from both interfaces.
Source
/ \
Plane1 Plane2
| |
A1 A2
| |
PE1----PE2
FIG3. Non-ECMP-mode MoFRR
3. The PE is part of a ring of PEs. This ring is attached to the
two planes via two aggregation routers. Each one being in a
different plane. PEs on the left side of the ring have a
clockwise path to the source while PEs on the right side of the
ring use an anti-clockwise path. Ring-mode MoFRR enables a PE to
receive the same PIM tree from both direction.
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Source
/ \
Plane1 Plane2
| |
A1 A2
| |
PE1----PE2
/ \
PE3 PE4
\ /
\ /
\ /
PE5
FIG4. Ring-mode MoFRR
4. Detecting Failures
Once the two paths are established, the next step is detecting a
failure on the primary path to know when to switch to the backup
path.
A first option consists of comparing the packets received on the
primary and secondary streams but only forwarding one of them -- the
first one received, no matter which interface it is received on.
Zero packet loss is possible for RTP-based streams.
A second option assumes a minimum packet rate for a given data
stream. If a packet is not received on the primary RPF within this
time frame, the router assumes primary path failure and switches to
the secondary RPF interface. 50msec switchover is possible.
A third option leverages the significant improvements of the IGP
convergence speed. When the primary path to the source is withdrawn
by the IGP, the MoFRR-enabled router switches over to the backup
path, the RPF interface is changed to the secondary RPF interface.
Since the secondary path is already in place, convergence times would
not include the time required to build a new tree and hence are
smaller. Realistic availability requirements (sub-second to sub-
200msec) should be possible.
5. ECMP-mode MoFRR
If the IGP installs two ECMP paths to the source and if the (S, G)
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PIM state is enabled for ECMP-Mode MoFRR, the router installs them as
primary RPF and secondary RPF. It sends a PIM join to both RPF
entries. Only packets receive from the primary RPF entry are
processed. Packets received from the secondary RPF are dropped
(equivalent to an RPF failure).
The selected primary RPF interface should be the same as if MoFRR
extension was not enabled.
If more than two ECMP paths exist, two are selected as primary and
secondary RPF interfaces. Information from the IGP link-state
topology could be leveraged to optimize this selection.
Note, MoFRR does not restrict the number of paths on which joins are
sent. Implementations may use as many paths as are configured.
6. Non-ECMP-mode MoFRR
SourceS
/ \
/ \
Backbone
| |
| |
| |
X--------N
Fig5. Non-ECMP-Mode MoFRR
X is configured for MoFRR for state (S, G)
R(X) is Xs RPF to S
N is a neighbor of X
R(N) is Ns RPF to S
xs represents the IGP metric from X to S
ns represents the IGP metric from N to S
xn represents the IGP metric from X to N
A router X configured for non-ECMP-mode MoFRR for (S, G) sends a
primary PIM join to its primary RPF R(X) and a secondary PIM Join to
a neighbor N if the following three conditions are met.
C1: xs < xn + ns
C2: ns < nx + xs
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C3: X cannot send a secondary join to N if N is the only member of the OIF list
The first condition ensures that N is not on the primary branch from
X to S.
The second condition ensures that X is not on the primary branch from
N to S.
These two conditions ensure that at least locally the two paths are
disjoint.
The third condition is required to break control-plane loops which
could occur in some scenarios.
For example in FIG3, if PE1 and PE2 have received an igmp request for
(S, G), they will both send a primary PIM join on their plane and a
secondary PIM join to the neighbor PE. If their receivers would
leave at the same time, it could be possible for the (S, G) states on
PE1 and PE2 to never get deleted as each PE refresh each other via
the secondary PIM joins (remember that a secondary PIM join is not
distinguishable from a primary PIM join. MoFRR does not require any
PIM protocol modification).
A control-plane loop occurs when two nodes keep a state forever due
to the secondary joins they send to each other. This forever
condition is not acceptable as no real receiver is connected to the
nodes (directly via IGMP or indirectly via PIM). Rule 3 prevents
this case as it prevents the mutual refresh of secondary joins and it
applies it in the specific case where there is no real receiver
connected.
6.1. Variation
Rule R3 can be removed if Rule 2 is restricted as follows:
R2p: ns < xs
This ensures that X only sends a secondary join to a neighbor N who
is strictly closer to the source than X is. By reciprocity, N will
thus never be able to send an sedondary join to the same source via
X. The strictly smaller than is key here.
Note that this non-ECMP-mode MoFRR variation does not support the
square topology and hence is less preferred.
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7. Ring Topologies
This will be documented in a future versions of the draft.
8. Keep It Simple Principle
Many Service Providers devise their topology such that PEs have
disjoint paths to the multicast sources. MoFRR leverages the
existence of these disjoint paths without any PIM protocol
modification. Interoperability testing is thus not required. In
such topologies, MoFRR only needs to be deployed on the PE devices.
Each PE device can be enabled one by one. PEs not enabled for MoFRR
do not see any change or degradation.
Multicast streams with Tight SLA requirements are often characterized
by a continuous high packet rate (SD video has a continuous
interpacket gap of ~ 3msec). MoFRR simply leverages the stream
characteristic to detect any failures along the primary branch and
switch-over on the secondary branch in a few 10s of msec.
9. Other Applications
While all the examples in this document show the MoFRR applicability
on PE devices, it is clear that MoFRR could be enabled on aggregation
or core routers.
MoFRR can be popular in Data Center network configurations. With the
advent of lower cost ethernet and increasing port density in routers,
there is more meshed connectivity than ever before. When using a
3-level access, distribution, and core layers in a Data Center, there
is a lot of inexpensive bandwidth connecting the layers. This will
lend itself to more opportunities for ECMP paths at multiple layers.
This allows for multiple layers of redundancy protecting link and
node failure at each layer with minimal redundancy cost.
Redundancy costs are reduced because only one packet is forwarded at
every link along the primary and secondary data paths so there is no
duplication of data on any link thereby providing make-before-break
protection at a very small cost.
The MoFRR behavior described for PIM are immediately applicable to
MLDP. Alternate methods to detect failures such as MPLS-OAM or BFD
may be considered.
The MoFRR principle may be applied to MVPNs.
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10. Security Considerations
There are no security considerations for this design other than what
is already in the main PIM specification [RFC4601].
11. Acknowledgments
The authors would like to thank John Zwiebel, Greg Shepherd and Dave
Oran for their review of the draft.
12. References
12.1. Normative References
[RFC5036] Andersson, L., Minei, I., and B. Thomas, "LDP
Specification", RFC 5036, October 2007.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
12.2. Informative References
[RFC4601] Fenner, B., Handley, M., Holbrook, H., and I. Kouvelas,
"Protocol Independent Multicast - Sparse Mode (PIM-SM):
Protocol Specification (Revised)", RFC 4601, August 2006.
Authors' Addresses
Apoorva Karan
Cisco Systems, Inc.
3750 Cisco Way
San Jose CA, 95134
USA
Email: apoorva@cisco.com
Clarence Filsfils
Cisco Systems, Inc.
De kleetlaan 6a
Diegem BRABANT 1831
Belgium
Email: cfilsfil@cisco.com
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Dino Farinacci
Cisco Systems, Inc.
425 East Tasman Drive
San Jose CA, 95134
USA
Email: dino@cisco.com
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